Review of step-pool hydrodynamics in mountain streams

Abstract

Step-pools are one of the major types of bed morphology prevalent in mountain streams. They have a unique flow structure as compared to low-gradient streams, in terms of large boundary elements and alternating super-critical and sub-critical flow conditions, which result in a non-uniform flow regime. Step-pools may also be constructed artificially to restore mountain incisions, and for creating close-to-nature fish passes. For hydraulic model development and various design considerations, the accurate prediction of flow phenomenon is required. This necessitates a detailed study of the turbulence phenomenon in natural step-pool reaches and its effect on the total flow. However, the influence of aerated conditions in step-pool hydrodynamics has not yet been adequately addressed. This paper presents a review of the mechanism of flow resistance and energy dissipation in step-pool mountain streams. Also, the significance of incorporating air entrainment in flow analysis, limitations and the way forward in modeling air–water flow in laboratory studies are discussed.

Flow Resistance in Very Rough Channels

“…This is in contrast to the second category where the boundary roughness length‐scale is as large or larger than the flow depth due to roughness elements such as immobile boulders, often found in steep mountain environments. The flow resistance for these channels is high and generally underpredicted by models developed for relatively smooth boundaries (Kalathil & Chandra, 2019; Powell, 2014; Shobe et al., 2021). Because the roughness boundary layer occupies much or all of the flow depth, the poor understanding of flow within this layer becomes especially problematic.…”

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Flow resistance in open channels, that is, the friction that the channel boundary exerts on the flow, is a fundamental hydraulic parameter defining many aspects of the behavior of a river channel (Kalathil & Chandra, 2019;Powell, 2014;Shobe et al, 2021). The roughness of the channel boundary relative to the depth of the flow is one of the primary influences on flow resistance (Cheng, 2017;Ferguson, 2007;Nitsche et al, 2012;Powell, 2014), and river channels can be broadly divided into two categories based on boundary roughness.The first category are those channels where the flow is significantly deeper than the scale of the boundary roughness, for example, deep relative to the characteristic grain size or bedform height.

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The influence of different morphological units on the turbulent flow characteristics in step-pool mountain streams

“…The commonly used flow friction factors such as Manning’s n and Chezy’s C cannot be applied here due to the non-uniform nature of flow at meso-scale. In step-pool mountain streams, the rational frictional coefficient to define flow resistance is the non-dimensional Darcy Weisbach friction factor 5 – 7 . Dedicated field and laboratory investigations of the step-pools are necessary to create a sufficient database for the development of accurate hydraulic models.…”

“…Primal research on step-pools has been largely limited to the analysis of bed morphology 14 – 17 , flow resistance 18 – 22 and sediment transport 23 , 24 by considering the step-pool reach as a single system. A detailed review on the hydrodynamics of step-pools in mountain streams is available in Kalathil and Chandra 7 .…”

Energy Loss in Steep Open Channels with Step-Pools

“…Many studies have been carried out in the past on natural step-pool streams using field data to understand: Changes in characteristics of a step-pool system as a function of flow rate [2,7]; standardizing geomorphic definitions of step height, step wavelength, active width, drainage area, slope and particle size [8]; entropy of step-pools, bed slope and bed friction [9]; the effect of sediment supply on the stability of step-pools [10] and step-pool structure [11]; design of scour depth and pool depth [12]; the effect of step-pools on ecology and flow resistance [13]. Recently, Kalathil and Chandra [14] provided an extensive review of work carried out on step-pool hydrodynamics in mountainous streams.…”